A good design is an obvious design.
Sometimes the most convenient design might look boring but it might be the most productive as well. How much does a website design affect your website's traffic?
Depending on the amount of traffic being the users on Mobile and slow Internet this number could variate a lot.
To answer this, think of yourself on a slow-ass Internet something like you'd get on a 2G Mobile Network. And browsing a website with a black background, with an overlay of white image and simple text in black over it. If the white image is significantly large in size to load in a short time, you're stuck with black text over a black background. The only way to actually see is by selecting the text.
Normally, a user would just leave the website unless you have some content he's really desperate for.
Now a website design also affects the Search Engine Optimisation, SEO of it. If you're not aware, Google's crawlers when exploring ranks the type of data they encounter according to various different factors.
Like if you have a step by step guide for something you should just bullet the steps and you'd be able to show your data in quick results in Google Search. That does not get you actual visitor but it does increase your website's authority in that search.
In a similar way, a simple design with proper tags and content is easily crawled by the search engines and hence getter better rankings.